#17 indicator (set) hash-tables

[Sam Steingold]

sometimes a hash table is a just a set,
i.e., not a general map, but a {0,1}-valued one.
I suggest a new hash table category, created via,
e.g., :TYPE :SET argument to MAKE-HASH-TABLE,
where there is NO values vector and GETHASH returns
either T,T or NIL,NIL.
It should be printed as
#S(HASH-TABLE <TEST> list-or-values)
and :INITIAL-CONTENT argument to MAKE-HASH-TABLE
should be a LIST (not an Association List).

[Sam Steingold]

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this HT will have no values vector which will save half the
memory.
also, access should be faster.